FIG. 5 is a fluid circuit diagram of a foaming apparatus 90 using a conventional mixing device.
In FIG. 5, a high-viscosity material contained in a container 91 is pressurized by a pump 92 and is transferred to a power mixer 94. A compressed gas in a gas cylinder 93 is adjusted to have a high pressure and is transferred to the power mixer 94.
The power mixer 94 is operated by rotating a motor M. The power mixer 94 mixes, under high pressure, a combination of the high-viscosity material and the gas supplied thereto to make the mixture thereof. The mixture of the high-viscosity material and the gas mixed by the power mixer 94 is discharged from a nozzle 96 through a pipe 95. As the gas, a nitrogen gas, a carbonic acid gas or air can be used. The foaming apparatus 90 arranged as mentioned above is used for, for example, applying a high-viscosity polymeric material such as a hotmelt adhesive (see Unexamined Japanese Patent Application Public Disclosure No. 63-264327).
A hotmelt adhesive comprises a thermoplastic polymer which is in a solid form at room temperature. The hotmelt adhesive is melted and fluidized by heating. When the hotmelt adhesive in a molten state is cooled to room temperature, the adhesive solidifies and exerts not only adhesive strength, but also exerts physical strength as a solid mass. In conventional foaming apparatuses for a hotmelt adhesive, a mixture of a gas and a molten hotmelt adhesive is cooled before occurrence of an escape of the gas and then, the mixture is foamed, with the gas being incorporated into the hotmelt adhesive by utilizing the strength of the adhesive which is exerted immediately after cooling.
In the above-mentioned conventional foaming apparatus 90, in order to mix the high-viscosity material and the gas, the high-viscosity material and the gas after heating must be transferred under high pressure to the power mixer 94. When the viscosity of the high-viscosity material is as high as, for example, 100,000 cps, it is considered that the internal pressure of the power mixer 94 is 100 kg/cm2 or more. Therefore, in order to supply the gas and the high-viscosity material to the power mixer 94 at the same time, the pressure of the gas needs to be higher than that of the high-viscosity material.
In the conventional foaming apparatus 90, the amount of gas is measured by controlling the flow rate of the gas. Note the illustrated mass flow controller (MFC). However, when the pressure of the gas is high, it is difficult to control the gas flow rate. Further, a minor error in the gas flow rate under high pressure becomes a major error under atmospheric pressure. For example, an error in the flow rate of 50 kg/cm2 becomes 50 times greater under atmospheric pressure. Therefore, in the conventional foaming apparatus 90, there are great variations in the mixing ratio of the high-viscosity material and the gas, so that foaming ratio cannot be stably conducted, thus making it difficult to achieve uniform foaming ratio.
Further, the high-pressure gas cylinder 93 for supplying the gas must be replaced when the compressed gas in the high-pressure gas cylinder 93 has been exhausted, so that maintenance becomes cumbersome. Further, for replacement of the high-pressure gas cylinder 93, another high-pressure gas cylinder 93 must be installed as a spare, so that a large space is required for installment of these gas cylinders. In addition, various measures must be taken from the viewpoint of safety, according to regulations for high-pressure gases. For these reasons, the conventional foaming apparatus is disadvantageous in terms of costs.